The present invention relates to an image coding apparatus for compressing and coding a video signal.
MPEG-2 is known as one of the high-efficiency video compression standards issued by Moving Picture Experts Group. In accordance with the MPEG-2 standard, each video frame (or field) is classified as an I, P or B frame. An I-frame is intra-frame coded using no reference frames. On the other hand, P- and B-frames are inter-frame predictively coded using some reference frames. Specifically, a P-frame is an abbreviation of a predictively coded frame, which is also called a “predicted frame”. A B-frame means a bidirectionally coded frame, which is also called a “bidirectionally predicted frame”. Each frame is also subdivided into a plurality of units called “macroblocks”, each of which is composed of 16×16 pixels. And each frame is coded by being subjected to discrete cosine transform (DCT) on a block-by-block basis, where each block is composed of 8×8 pixels.
According to a technique disclosed in Japanese Laid-Open Publication No. 9-271026, when video with a relatively high coding complexity is input, the resolution of succeeding video signals is decreased to constantly control the number of binary bits generated per unit time at a predetermined number or less. This countermeasure is taken when either the product of a quantization scale (i.e., the magnitude of a unit quantization step) and the number of bits generated or an occupied buffer capacity is about to reach a certain limit. Thus, if the number of bits generated is going to exceed the predetermined number, the resolution must be decreased even if the input video signal contains a lot of high-frequency components. Accordingly, deterioration of image quality is unavoidable in such a situation.